Arkham City was released on October 18th, 2011. Three years ago. Despite that, players are still finding stuff hidden in the game.

Last week, a mysterious YouTube channel uploaded a small clip titled "Arkham City Secret?" In it, a character named Calendar Man could be heard doing dialogue that players had never encountered before—but the clip blacked out before players could hear everything.

Two days later, Batman Arkham Videos seemed to crack the mystery. Turns out, if you set your PC to the date December 13th, 2004, you trigger special Calendar Man dialogue—which you can see in the clip above. The date seems random, but players speculate that it's tied to a very special date. That's the year that the developer behind the game, Rocksteady, was founded after all.

This is interesting for a number of reasons. Might there be more easter eggs that nobody has found in Arkham City? Was the uploader of the original video Rocksteady itself? Players seem to think so. And is Calendar Man teasing the end of the franchise, or the death of Batman? Is he teasing a specific villain in the upcoming game? Who knows! What's clear is that it's cool that players found something new in the game, even if they might have needed a nudge from Rocksteady.